JDO : Data Federation

By default JDO provides a PersistenceManagerFactory (PMF) to represent a datastore.
DataNucleus extends this to allow for a PMF to represent multiple datastores. This is intended for
use where you have a data model for an application and maybe some classes are persisted into
a different datastore.
Note that this is work-in-progress and only tested for basic persist/retrieve operations
using different schemas of the same datastore.
Obviously if you have relations between one object in one datastore and another object in another
datastore you cannot have foreign-keys (or equivalent).

Defining Primary and Secondary Datastores

You could specify the datastores to be used for the PMF like this. Here we have
datanucleus.properties defining the primary datastore.